Mice Capades

Potential use of rodenticides on the mice of the Farallon Islands divides environmentalists

Sean Van Sommeran, a shark researcher based in Santa Cruz, believes rodenticides applied at the Farallones could remain in the environment for long periods.

"They're pretending this won't have residual impacts," says Van Sommeran, the founder and director of the Pelagic Shark Research Foundation. "It's going to affect seabirds and marine mammals. It's just going to be one more addition to the contaminants already in the water." He believes the rodenticide could migrate through the food web and eventually contaminate large predators—like great white sharks, the core of Van Sommeran's research—much the way that heavy metals find their way into sharks, swordfish and tuna.

There is little doubt that some birds—especially omnivorous western gulls—will eat the pellets and die. But Cordell says casualties could be minimized by scaring away the birds during the poisoning effort. Hazing methods—like using loud explosives and laser pointers to scatter flocks of gulls—have been tested already and proven effective at the islands. Owls, liable to suffer the consequences of eating poisoned mice, would need to be trapped and relocated during the eradication effort, Cordell says.

Eliminating the mice will benefit more than just petrels, says Brad Keitt of Island Conservation, a group based in Santa Cruz. "Removing invasive species has had incredible benefits to islands around the world," he says. At the Farallones, Keitt says, "the driving issue is to restore the balance of the ecosystem."

The Farallon Islands have seen non-native species come and go before. The islands were first visited by Russian sailors in the early 1800s, but it's believed by scientists who have genetically examined the islands' mice that the rodents were brought later in the century, from mainland American stock. Around the same time, rabbits were released on Southeast Farallon Island. Hundreds of them were still living there in 1971, as were several feral cats, when a scientist named David Ainley first set foot on the island.

"There was a lot of junk out there—sheds and garbage and things," says Ainley, a Marin City resident who previously worked for the Point Reyes Bird Observatory and, through the 1970s, spent about half his life living on the island. "We got that all cleaned out."

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Jim Tietz/Point Blue

NOT SO CUTE The Farallon mice knock over dishes, defecate on the dinner table and rustle through sleeping people’s hair.

Ainley helped direct a focused trapping and shooting effort that successfully eliminated the rabbits. Three cats, he says, were captured and sent to the mainland. The mice, however, remained. In fact, removing the rabbits meant more food for the mice, especially the seeds of the many grasses that consequently thrived unchecked. The mouse population soared higher than ever.

"Poisoning is the only chance to get rid of the mice," Ainley says. But mice, he says, are not easy animals to eradicate, both because they are small and easily able to remain unseen and because they reproduce prolifically. Southeast Farallon Island, at high tide, is roughly 60 acres, Ainley says. "There are infinite cracks and holes that they can hide in."

Every winter, the Farallones' mouse population plummets. Pyle explains that the first rains cause millions of small seeds scattered about the islands to germinate. This leaves the mice with nothing to eat. On top of that, winter rainfall tends to flood out their burrows, driving tens of thousands of starving mice into the cold open air.

"They come out of their holes and go wandering around eating each other," Pyle says.

He feels that eradicating the mice would not just benefit birds but would eliminate immeasurable rodent suffering. So many mice starve each winter on the Farallones that for several months, from March to June, resident researchers don't see a sign of the animals. Pyle guesses the mouse population bottoms out at perhaps 100 scrawny survivors in the early spring.

"Then the numbers start climbing, and by October it's mayhem again," he says.

Any poisoning effort would take advantage of this population cycle by hitting the mice while their numbers are down. The Fish and Wildlife Service has proposed the poisoning to take place in November of 2014, although the service is still considering its options and will release a final environmental impact statement this spring.